1PRCTL(2)                   Linux Programmer's Manual                  PRCTL(2)


6       prctl - operations on a process


9       #include <sys/prctl.h>
11       int prctl(int option, unsigned long arg2, unsigned long arg3,
12                 unsigned long arg4, unsigned long arg5);


15       prctl()  is  called  with  a first argument describing what to do (with
16       values defined in <linux/prctl.h>), and further arguments with  a  sig‐
17       nificance depending on the first one.  The first argument can be:
19       PR_CAP_AMBIENT (since Linux 4.3)
20              Reads  or  changes  the  ambient  capability  set of the calling
21              thread, according to the value of arg2, which must be one of the
22              following:
24              PR_CAP_AMBIENT_RAISE
25                     The  capability specified in arg3 is added to the ambient
26                     set.  The specified capability must already be present in
27                     both  the  permitted  and  the  inheritable  sets  of the
28                     process.   This  operation  is  not  permitted   if   the
29                     SECBIT_NO_CAP_AMBIENT_RAISE securebit is set.
31              PR_CAP_AMBIENT_LOWER
32                     The  capability  specified  in  arg3  is removed from the
33                     ambient set.
35              PR_CAP_AMBIENT_IS_SET
36                     The prctl() call returns 1 if the capability in  arg3  is
37                     in the ambient set and 0 if it is not.
39              PR_CAP_AMBIENT_CLEAR_ALL
40                     All  capabilities  will  be removed from the ambient set.
41                     This operation requires setting arg3 to zero.
43              In all of the above operations, arg4 and arg5 must be  specified
44              as 0.
46              Higher-level  interfaces  layered on top of the above operations
47              are  provided  in  the  libcap(3)  library  in   the   form   of
48              cap_get_ambient(3),   cap_set_ambient(3),   and  cap_reset_ambi‐
49              ent(3).
51       PR_CAPBSET_READ (since Linux 2.6.25)
52              Return (as the function result) 1 if the capability specified in
53              arg2 is in the calling thread's capability bounding set, or 0 if
54              it  is  not.   (The  capability   constants   are   defined   in
55              <linux/capability.h>.)   The  capability  bounding  set dictates
56              whether the process can receive the capability through a  file's
57              permitted capability set on a subsequent call to execve(2).
59              If  the capability specified in arg2 is not valid, then the call
60              fails with the error EINVAL.
62              A higher-level interface layered on top  of  this  operation  is
63              provided   in   the   libcap(3)   library   in   the   form   of
64              cap_get_bound(3).
66       PR_CAPBSET_DROP (since Linux 2.6.25)
67              If the calling thread has the CAP_SETPCAP capability within  its
68              user  namespace, then drop the capability specified by arg2 from
69              the calling thread's capability bounding set.  Any  children  of
70              the calling thread will inherit the newly reduced bounding set.
72              The  call fails with the error: EPERM if the calling thread does
73              not have the CAP_SETPCAP; EINVAL if arg2 does  not  represent  a
74              valid capability; or EINVAL if file capabilities are not enabled
75              in the kernel, in which case bounding sets are not supported.
77              A higher-level interface layered on top  of  this  operation  is
78              provided   in   the   libcap(3)   library   in   the   form   of
79              cap_drop_bound(3).
81       PR_SET_CHILD_SUBREAPER (since Linux 3.4)
82              If arg2 is nonzero, set the "child subreaper" attribute  of  the
83              calling process; if arg2 is zero, unset the attribute.
85              A subreaper fulfills the role of init(1) for its descendant pro‐
86              cesses.  When a process becomes orphaned  (i.e.,  its  immediate
87              parent  terminates), then that process will be reparented to the
88              nearest still living ancestor subreaper.  Subsequently, calls to
89              getppid() in the orphaned process will now return the PID of the
90              subreaper process, and when the orphan  terminates,  it  is  the
91              subreaper process that will receive a SIGCHLD signal and will be
92              able to wait(2) on the process to discover its termination  sta‐
93              tus.
95              The  setting of the "child subreaper" attribute is not inherited
96              by children created by fork(2) and  clone(2).   The  setting  is
97              preserved across execve(2).
99              Establishing a subreaper process is useful in session management
100              frameworks where a hierarchical group of processes is managed by
101              a  subreaper  process  that needs to be informed when one of the
102              processes—for example, a double-forked  daemon—terminates  (per‐
103              haps  so that it can restart that process).  Some init(1) frame‐
104              works (e.g., systemd(1)) employ a subreaper process for  similar
105              reasons.
107       PR_GET_CHILD_SUBREAPER (since Linux 3.4)
108              Return the "child subreaper" setting of the caller, in the loca‐
109              tion pointed to by (int *) arg2.
111       PR_SET_DUMPABLE (since Linux 2.3.20)
112              Set the state of the "dumpable" flag, which  determines  whether
113              core dumps are produced for the calling process upon delivery of
114              a signal whose default behavior is to produce a core dump.
116              In kernels up to and including 2.6.12, arg2  must  be  either  0
117              (SUID_DUMP_DISABLE,    process    is    not   dumpable)   or   1
118              (SUID_DUMP_USER, process is dumpable).  Between  kernels  2.6.13
119              and  2.6.17,  the  value  2 was also permitted, which caused any
120              binary which normally would not be dumped to be dumped  readable
121              by  root  only;  for  security  reasons,  this  feature has been
122              removed.    (See   also   the   description   of   /proc/sys/fs/
123              suid_dumpable in proc(5).)
125              Normally,  this  flag  is set to 1.  However, it is reset to the
126              current value contained in the  file  /proc/sys/fs/suid_dumpable
127              (which  by  default  has  the value 0), in the following circum‐
128              stances:
130              *  The process's effective user or group ID is changed.
132              *  The process's filesystem user or group  ID  is  changed  (see
133                 credentials(7)).
135              *  The  process executes (execve(2)) a set-user-ID or set-group-
136                 ID program, resulting in a change  of  either  the  effective
137                 user ID or the effective group ID.
139              *  The  process  executes  (execve(2))  a  program that has file
140                 capabilities (see capabilities(7)), but only if the permitted
141                 capabilities  gained  exceed  those already permitted for the
142                 process.
144              Processes  that  are  not  dumpable  can  not  be  attached  via
145              ptrace(2) PTRACE_ATTACH; see ptrace(2) for further details.
147              If  a  process  is  not  dumpable, the ownership of files in the
148              process's /proc/[pid] directory  is  affected  as  described  in
149              proc(5).
151       PR_GET_DUMPABLE (since Linux 2.3.20)
152              Return (as the function result) the current state of the calling
153              process's dumpable flag.
155       PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
156              Set the endian-ness of the calling process to the value given in
157              arg2,  which  should  be  one  of  the following: PR_ENDIAN_BIG,
158              PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE (PowerPC pseudo little
159              endian).
161       PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
162              Return  the  endian-ness of the calling process, in the location
163              pointed to by (int *) arg2.
165       PR_SET_FP_MODE (since Linux 4.0, only on MIPS)
166              On the MIPS architecture, user-space code can be built using  an
167              ABI  which  permits  linking with code that has more restrictive
168              floating-point (FP) requirements.  For example, user-space  code
169              may  be  built  to  target the O32 FPXX ABI and linked with code
170              built for either one of the more restrictive FP32 or FP64  ABIs.
171              When more restrictive code is linked in, the overall requirement
172              for the process is to use the  more  restrictive  floating-point
173              mode.
175              Because the kernel has no means of knowing in advance which mode
176              the process should be executed in, and  because  these  restric‐
177              tions   can  change  over  the  lifetime  of  the  process,  the
178              PR_SET_FP_MODE operation is provided to  allow  control  of  the
179              floating-point mode from user space.
181              The  (unsigned  int)  arg2 argument is a bit mask describing the
182              floating-point mode used:
184              PR_FP_MODE_FR
185                     When this bit is unset (so called FR=0 or FR0 mode),  the
186                     32  floating-point registers are 32 bits wide, and 64-bit
187                     registers are represented as a pair of  registers  (even-
188                     and  odd-  numbered, with the even-numbered register con‐
189                     taining the lower 32 bits, and the odd-numbered  register
190                     containing the higher 32 bits).
192                     When  this  bit  is  set  (on supported hardware), the 32
193                     floating-point registers are 64 bits wide (so called FR=1
194                     or  FR1  mode).   Note  that  modern MIPS implementations
195                     (MIPS R6 and newer) support FR=1 mode only.
197                     Applications that use the O32 FP32 ABI can  operate  only
198                     when  this  bit  is unset (FR=0; or they can be used with
199                     FRE enabled, see below).  Applications that use  the  O32
200                     FP64  ABI (and the O32 FP64A ABI, which exists to provide
201                     the ability to  operate  with  existing  FP32  code;  see
202                     below)  can  operate  only  when  this bit is set (FR=1).
203                     Applications that use the O32 FPXX ABI can  operate  with
204                     either FR=0 or FR=1.
206              PR_FP_MODE_FRE
207                     Enable  emulation  of  32-bit  floating-point mode.  When
208                     this mode is enabled, it emulates  32-bit  floating-point
209                     operations by raising a reserved-instruction exception on
210                     every instruction that uses 32-bit formats and the kernel
211                     then  handles  the instruction in software.  (The problem
212                     lies in the discrepancy of handling  odd-numbered  regis‐
213                     ters  which are the high 32 bits of 64-bit registers with
214                     even numbers in FR=0 mode and the lower 32-bit  parts  of
215                     odd-numbered  64-bit  registers  in FR=1 mode.)  Enabling
216                     this bit is necessary when code with  the  O32  FP32  ABI
217                     should  operate with code with compatible the O32 FPXX or
218                     O32 FP64A ABIs (which require FR=1 FPU mode) or  when  it
219                     is  executed  on  newer  hardware (MIPS R6 onwards) which
220                     lacks FR=0 mode support when a binary with the  FP32  ABI
221                     is used.
223                     Note  that  this mode makes sense only when the FPU is in
224                     64-bit mode (FR=1).
226                     Note that the use of emulation inherently has a  signifi‐
227                     cant performance hit and should be avoided if possible.
229              In  the  N32/N64 ABI, 64-bit floating-point mode is always used,
230              so FPU emulation is not required and the FPU always operates  in
231              FR=1 mode.
233              This  option  is  mainly  intended for use by the dynamic linker
234              (ld.so(8)).
236              The arguments arg3, arg4, and arg5 are ignored.
238       PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
239              Return (as the function result) the current floating-point  mode
240              (see the description of PR_SET_FP_MODE for details).
242              On  success,  the  call  returns a bit mask which represents the
243              current floating-point mode.
245              The arguments arg2, arg3, arg4, and arg5 are ignored.
247       PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
248              Set  floating-point  emulation  control  bits  to  arg2.    Pass
249              PR_FPEMU_NOPRINT  to  silently  emulate floating-point operation
250              accesses, or PR_FPEMU_SIGFPE to not emulate floating-point oper‐
251              ations and send SIGFPE instead.
253       PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
254              Return  floating-point  emulation  control bits, in the location
255              pointed to by (int *) arg2.
257       PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
258              Set   floating-point   exception    mode    to    arg2.     Pass
259              PR_FP_EXC_SW_ENABLE  to  use  FPEXC  for  FP  exception enables,
260              PR_FP_EXC_DIV for floating-point divide by  zero,  PR_FP_EXC_OVF
261              for  floating-point  overflow,  PR_FP_EXC_UND for floating-point
262              underflow,  PR_FP_EXC_RES  for  floating-point  inexact  result,
263              PR_FP_EXC_INV     for    floating-point    invalid    operation,
264              PR_FP_EXC_DISABLED for FP exceptions disabled,  PR_FP_EXC_NONRE‐
265              COV for async nonrecoverable exception mode, PR_FP_EXC_ASYNC for
266              async recoverable exception mode, PR_FP_EXC_PRECISE for  precise
267              exception mode.
269       PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
270              Return floating-point exception mode, in the location pointed to
271              by (int *) arg2.
273       PR_SET_KEEPCAPS (since Linux 2.2.18)
274              Set the state of the calling thread's "keep capabilities"  flag.
275              The  effect  of this flag is described in capabilities(7).  arg2
276              must be either 0 (clear the flag) or  1  (set  the  flag).   The
277              "keep capabilities" value will be reset to 0 on subsequent calls
278              to execve(2).
280       PR_GET_KEEPCAPS (since Linux 2.2.18)
281              Return (as the function result) the current state of the calling
282              thread's  "keep  capabilities"  flag.  See capabilities(7) for a
283              description of this flag.
285       PR_MCE_KILL (since Linux 2.6.32)
286              Set the machine check memory  corruption  kill  policy  for  the
287              calling  thread.  If arg2 is PR_MCE_KILL_CLEAR, clear the thread
288              memory corruption kill policy and use the  system-wide  default.
289              (The system-wide default is defined by /proc/sys/vm/memory_fail‐
290              ure_early_kill; see proc(5).)  If arg2 is PR_MCE_KILL_SET, use a
291              thread-specific  memory  corruption  kill policy.  In this case,
292              arg3   defines   whether    the    policy    is    early    kill
293              (PR_MCE_KILL_EARLY),  late  kill (PR_MCE_KILL_LATE), or the sys‐
294              tem-wide default (PR_MCE_KILL_DEFAULT).  Early kill  means  that
295              the  thread  receives a SIGBUS signal as soon as hardware memory
296              corruption is detected inside its address space.  In  late  kill
297              mode,  the  process  is killed only when it accesses a corrupted
298              page.  See sigaction(2) for more information on the SIGBUS  sig‐
299              nal.  The policy is inherited by children.  The remaining unused
300              prctl() arguments must be zero for future compatibility.
302       PR_MCE_KILL_GET (since Linux 2.6.32)
303              Return (as the function result) the current per-process  machine
304              check kill policy.  All unused prctl() arguments must be zero.
306       PR_SET_MM (since Linux 3.3)
307              Modify  certain kernel memory map descriptor fields of the call‐
308              ing process.  Usually these fields are set  by  the  kernel  and
309              dynamic loader (see ld.so(8) for more information) and a regular
310              application should not use this  feature.   However,  there  are
311              cases,  such  as  self-modifying programs, where a program might
312              find it useful to change its own memory map.
314              The calling process must have the  CAP_SYS_RESOURCE  capability.
315              The  value  in arg2 is one of the options below, while arg3 pro‐
316              vides a new value for the option.  The arg4 and  arg5  arguments
317              must be zero if unused.
319              Before  Linux 3.10, this feature is available only if the kernel
320              is built with the CONFIG_CHECKPOINT_RESTORE option enabled.
322              PR_SET_MM_START_CODE
323                     Set the address above which the  program  text  can  run.
324                     The  corresponding  memory area must be readable and exe‐
325                     cutable, but not writable or shareable  (see  mprotect(2)
326                     and mmap(2) for more information).
328              PR_SET_MM_END_CODE
329                     Set  the  address  below  which the program text can run.
330                     The corresponding memory area must be readable  and  exe‐
331                     cutable, but not writable or shareable.
333              PR_SET_MM_START_DATA
334                     Set the address above which initialized and uninitialized
335                     (bss) data are placed.   The  corresponding  memory  area
336                     must  be  readable  and  writable,  but not executable or
337                     shareable.
339              PR_SET_MM_END_DATA
340                     Set the address below which initialized and uninitialized
341                     (bss)  data  are  placed.   The corresponding memory area
342                     must be readable and  writable,  but  not  executable  or
343                     shareable.
345              PR_SET_MM_START_STACK
346                     Set  the  start  address of the stack.  The corresponding
347                     memory area must be readable and writable.
349              PR_SET_MM_START_BRK
350                     Set the address above  which  the  program  heap  can  be
351                     expanded  with  brk(2) call.  The address must be greater
352                     than the ending address of the current program data  seg‐
353                     ment.   In  addition,  the combined size of the resulting
354                     heap and the size of the data segment  can't  exceed  the
355                     RLIMIT_DATA resource limit (see setrlimit(2)).
357              PR_SET_MM_BRK
358                     Set  the  current brk(2) value.  The requirements for the
359                     address are  the  same  as  for  the  PR_SET_MM_START_BRK
360                     option.
362              The following options are available since Linux 3.5.
364              PR_SET_MM_ARG_START
365                     Set  the  address above which the program command line is
366                     placed.
368              PR_SET_MM_ARG_END
369                     Set the address below which the program command  line  is
370                     placed.
372              PR_SET_MM_ENV_START
373                     Set  the  address  above which the program environment is
374                     placed.
376              PR_SET_MM_ENV_END
377                     Set the address below which the  program  environment  is
378                     placed.
380                     The     address    passed    with    PR_SET_MM_ARG_START,
381                     PR_SET_MM_ARG_END,        PR_SET_MM_ENV_START,        and
382                     PR_SET_MM_ENV_END  should belong to a process stack area.
383                     Thus, the corresponding memory  area  must  be  readable,
384                     writable,  and  (depending  on  the kernel configuration)
385                     have the MAP_GROWSDOWN attribute set (see mmap(2)).
387              PR_SET_MM_AUXV
388                     Set a new auxiliary vector.   The  arg3  argument  should
389                     provide  the address of the vector.  The arg4 is the size
390                     of the vector.
392              PR_SET_MM_EXE_FILE
393                     Supersede the /proc/pid/exe symbolic link with a new  one
394                     pointing  to a new executable file identified by the file
395                     descriptor provided in arg3 argument.  The file  descrip‐
396                     tor should be obtained with a regular open(2) call.
398                     To  change  the  symbolic  link,  one  needs to unmap all
399                     existing executable memory areas, including those created
400                     by the kernel itself (for example the kernel usually cre‐
401                     ates at least one executable  memory  area  for  the  ELF
402                     .text section).
404                     In  Linux  4.9 and earlier, the PR_SET_MM_EXE_FILE opera‐
405                     tion can be performed only once in a process's  lifetime;
406                     attempting to perform the operation a second time results
407                     in the error EPERM.  This restriction  was  enforced  for
408                     security  reasons that were subsequently deemed specious,
409                     and the restriction was removed  in  Linux  4.10  because
410                     some user-space applications needed to perform this oper‐
411                     ation more than once.
413              The following options are available since Linux 3.18.
415              PR_SET_MM_MAP
416                     Provides one-shot access to all the addresses by  passing
417                     in a struct prctl_mm_map (as defined in <linux/prctl.h>).
418                     The arg4 argument should provide the size of the struct.
420                     This feature is available only if  the  kernel  is  built
421                     with the CONFIG_CHECKPOINT_RESTORE option enabled.
423              PR_SET_MM_MAP_SIZE
424                     Returns  the  size  of the struct prctl_mm_map the kernel
425                     expects.  This allows user space  to  find  a  compatible
426                     struct.   The  arg4  argument  should  be a pointer to an
427                     unsigned int.
429                     This feature is available only if  the  kernel  is  built
430                     with the CONFIG_CHECKPOINT_RESTORE option enabled.
433              Enable  or disable kernel management of Memory Protection eXten‐
434              sions (MPX) bounds tables.  The arg2, arg3, arg4, and arg5 argu‐
435              ments must be zero.
437              MPX  is  a  hardware-assisted  mechanism  for  performing bounds
438              checking on pointers.  It consists of a set of registers storing
439              bounds  information  and  a  set of special instruction prefixes
440              that tell the CPU on which  instructions  it  should  do  bounds
441              enforcement.   There  is a limited number of these registers and
442              when there are more pointers than registers, their contents must
443              be  "spilled"  into  a  set  of tables.  These tables are called
444              "bounds tables" and the MPX prctl() operations  control  whether
445              the kernel manages their allocation and freeing.
447              When management is enabled, the kernel will take over allocation
448              and freeing of the bounds tables.  It does this by trapping  the
449              #BR exceptions that result at first use of missing bounds tables
450              and instead of delivering the exception to user space, it  allo‐
451              cates  the  table  and  populates  the bounds directory with the
452              location of the new table.  For freeing, the  kernel  checks  to
453              see  if  bounds tables are present for memory which is not allo‐
454              cated, and frees them if so.
456              Before enabling MPX management  using  PR_MPX_ENABLE_MANAGEMENT,
457              the  application  must  first have allocated a user-space buffer
458              for the bounds directory and placed the location of that  direc‐
459              tory in the bndcfgu register.
461              These  calls  fail  if  the  CPU or kernel does not support MPX.
462              Kernel support for MPX is enabled via  the  CONFIG_X86_INTEL_MPX
463              configuration  option.   You  can check whether the CPU supports
464              MPX by looking for the 'mpx' CPUID bit, like with the  following
465              command:
467                  cat /proc/cpuinfo | grep ' mpx '
469              A  thread  may  not switch in or out of long (64-bit) mode while
470              MPX is enabled.
472              All threads in a process are affected by these calls.
474              The child of a fork(2) inherits the  state  of  MPX  management.
475              During  execve(2),  MPX  management  is  reset  to a state as if
476              PR_MPX_DISABLE_MANAGEMENT had been called.
478              For further information on Intel MPX, see the kernel source file
479              Documentation/x86/intel_mpx.txt.
481       PR_SET_NAME (since Linux 2.6.9)
482              Set the name of the calling thread, using the value in the loca‐
483              tion pointed to by (char *) arg2.  The name  can  be  up  to  16
484              bytes long, including the terminating null byte.  (If the length
485              of the string, including the terminating null byte,  exceeds  16
486              bytes,  the  string  is  silently  truncated.)  This is the same
487              attribute  that  can  be  set  via   pthread_setname_np(3)   and
488              retrieved  using  pthread_getname_np(3).  The attribute is like‐
489              wise accessible via /proc/self/task/[tid]/comm, where tid is the
490              name of the calling thread.
492       PR_GET_NAME (since Linux 2.6.11)
493              Return  the name of the calling thread, in the buffer pointed to
494              by (char *) arg2.  The buffer should allow space for  up  to  16
495              bytes; the returned string will be null-terminated.
497       PR_SET_NO_NEW_PRIVS (since Linux 3.5)
498              Set  the calling thread's no_new_privs attribute to the value in
499              arg2.  With no_new_privs set to 1,  execve(2)  promises  not  to
500              grant  privileges  to  do anything that could not have been done
501              without the execve(2) call (for example, rendering the set-user-
502              ID  and  set-group-ID mode bits, and file capabilities non-func‐
503              tional).  Once set, this the no_new_privs  attribute  cannot  be
504              unset.   The  setting of this attribute is inherited by children
505              created by fork(2) and clone(2), and preserved across execve(2).
507              Since Linux 4.10, the value of a thread's no_new_privs attribute
508              can be viewed via the NoNewPrivs field in the /proc/[pid]/status
509              file.
511              For more information, see  the  kernel  source  file  Documenta‐
512              tion/userspace-api/no_new_privs.rst        (or        Documenta‐
513              tion/prctl/no_new_privs.txt before Linux 4.13).  See  also  sec‐
514              comp(2).
516       PR_GET_NO_NEW_PRIVS (since Linux 3.5)
517              Return  (as  the  function result) the value of the no_new_privs
518              attribute for the calling thread.  A value of  0  indicates  the
519              regular  execve(2)  behavior.   A value of 1 indicates execve(2)
520              will operate in the privilege-restricting mode described above.
522       PR_SET_PDEATHSIG (since Linux 2.1.57)
523              Set the parent-death signal  of  the  calling  process  to  arg2
524              (either  a  signal value in the range 1..maxsig, or 0 to clear).
525              This is the signal that the calling process will  get  when  its
526              parent dies.
528              Warning:  the  "parent"  in  this  case  is considered to be the
529              thread that created this process.  In other  words,  the  signal
530              will  be  sent  when  that  thread terminates (via, for example,
531              pthread_exit(3)), rather than after all of the  threads  in  the
532              parent process terminate.
534              The  parent-death  signal is sent upon subsequent termination of
535              the parent thread and also upon termination  of  each  subreaper
536              process (see the description of PR_SET_CHILD_SUBREAPER above) to
537              which the caller is  subsequently  reparented.   If  the  parent
538              thread  and  all  ancestor subreapers have already terminated by
539              the time of the PR_SET_PDEATHSIG operation, then no parent-death
540              signal is sent to the caller.
542              The parent-death signal is process-directed (see signal(7)) and,
543              if the child installs a handler using the  sigaction(2)  SA_SIG‐
544              INFO  flag,  the  si_pid  field of the siginfo_t argument of the
545              handler contains the PID of the terminating parent process.
547              The parent-death signal setting is cleared for the  child  of  a
548              fork(2).   It is also (since Linux 2.4.36 / 2.6.23) cleared when
549              executing a set-user-ID or set-group-ID binary, or a binary that
550              has  associated  capabilities  (see capabilities(7)); otherwise,
551              this value is preserved across execve(2).
553       PR_GET_PDEATHSIG (since Linux 2.3.15)
554              Return the current value of the parent process death signal,  in
555              the location pointed to by (int *) arg2.
557       PR_SET_PTRACER (since Linux 3.4)
558              This is meaningful only when the Yama LSM is enabled and in mode
559              1   ("restricted    ptrace",    visible    via    /proc/sys/ker‐
560              nel/yama/ptrace_scope).   When  a "ptracer process ID" is passed
561              in arg2, the caller is declaring that the  ptracer  process  can
562              ptrace(2)  the  calling  process  as if it were a direct process
563              ancestor.  Each PR_SET_PTRACER operation replaces  the  previous
564              "ptracer process ID".  Employing PR_SET_PTRACER with arg2 set to
565              0  clears  the  caller's  "ptracer  process  ID".   If  arg2  is
566              PR_SET_PTRACER_ANY,  the  ptrace restrictions introduced by Yama
567              are effectively disabled for the calling process.
569              For further information, see the kernel source  file  Documenta‐
570              tion/admin-guide/LSM/Yama.rst       (or      Documentation/secu‐
571              rity/Yama.txt before Linux 4.13).
573       PR_SET_SECCOMP (since Linux 2.6.23)
574              Set the secure computing (seccomp) mode for the calling  thread,
575              to limit the available system calls.  The more recent seccomp(2)
576              system  call  provides  a  superset  of  the  functionality   of
577              PR_SET_SECCOMP.
579              The  seccomp  mode is selected via arg2.  (The seccomp constants
580              are defined in <linux/seccomp.h>.)
582              With arg2 set to SECCOMP_MODE_STRICT, the only system calls that
583              the  thread is permitted to make are read(2), write(2), _exit(2)
584              (but not exit_group(2)), and sigreturn(2).  Other  system  calls
585              result  in the delivery of a SIGKILL signal.  Strict secure com‐
586              puting mode is useful for number-crunching applications that may
587              need to execute untrusted byte code, perhaps obtained by reading
588              from a pipe or socket.  This operation is available only if  the
589              kernel is configured with CONFIG_SECCOMP enabled.
591              With arg2 set to SECCOMP_MODE_FILTER (since Linux 3.5), the sys‐
592              tem calls allowed are defined by a pointer to a Berkeley  Packet
593              Filter  passed  in  arg3.   This argument is a pointer to struct
594              sock_fprog; it can be designed to filter arbitrary system  calls
595              and  system  call arguments.  This mode is available only if the
596              kernel is configured with CONFIG_SECCOMP_FILTER enabled.
598              If SECCOMP_MODE_FILTER filters permit fork(2), then the  seccomp
599              mode  is  inherited by children created by fork(2); if execve(2)
600              is  permitted,  then  the  seccomp  mode  is  preserved   across
601              execve(2).  If the filters permit prctl() calls, then additional
602              filters can be added; they are run in order until the first non-
603              allow result is seen.
605              For  further  information, see the kernel source file Documenta‐
606              tion/userspace-api/seccomp_filter.rst       (or       Documenta‐
607              tion/prctl/seccomp_filter.txt before Linux 4.13).
609       PR_GET_SECCOMP (since Linux 2.6.23)
610              Return (as the function result) the secure computing mode of the
611              calling thread.  If the caller is not in secure computing  mode,
612              this operation returns 0; if the caller is in strict secure com‐
613              puting mode, then the prctl() call will cause a  SIGKILL  signal
614              to be sent to the process.  If the caller is in filter mode, and
615              this system call is allowed by the seccomp filters,  it  returns
616              2; otherwise, the process is killed with a SIGKILL signal.  This
617              operation is available only if the  kernel  is  configured  with
618              CONFIG_SECCOMP enabled.
620              Since  Linux  3.8,  the  Seccomp field of the /proc/[pid]/status
621              file provides a method of obtaining the same information,  with‐
622              out the risk that the process is killed; see proc(5).
624       PR_SET_SECUREBITS (since Linux 2.6.26)
625              Set  the  "securebits"  flags of the calling thread to the value
626              supplied in arg2.  See capabilities(7).
628       PR_GET_SECUREBITS (since Linux 2.6.26)
629              Return (as the function result) the "securebits"  flags  of  the
630              calling thread.  See capabilities(7).
632       PR_GET_SPECULATION_CTRL (since Linux 4.17)
633              Return  (as  the  function  result) the state of the speculation
634              misfeature specified in arg2.   Currently,  the  only  permitted
635              value  for  this argument is PR_SPEC_STORE_BYPASS (otherwise the
636              call fails with the error ENODEV).
638              The return value uses bits 0-3 with the following meaning:
640              PR_SPEC_PRCTL
641                     Mitigation can be controlled per thread by  PR_SET_SPECU‐
642                     LATION_CTRL
644              PR_SPEC_ENABLE
645                     The  speculation  feature  is enabled, mitigation is dis‐
646                     abled.
648              PR_SPEC_DISABLE
649                     The  speculation  feature  is  disabled,  mitigation   is
650                     enabled
652              PR_SPEC_FORCE_DISABLE
653                     Same as PR_SPEC_DISABLE but cannot be undone.
655              If  all bits are 0, then the CPU is not affected by the specula‐
656              tion misfeature.
658              If PR_SPEC_PRCTL is set, then per-thread control of the  mitiga‐
659              tion is available.  If not set, prctl() for the speculation mis‐
660              feature will fail.
662              The arg3, arg4, and arg5 arguments must be specified as 0;  oth‐
663              erwise the call fails with the error EINVAL.
665       PR_SET_SPECULATION_CTRL (since Linux 4.17)
666              Sets  the state of the speculation misfeature specified in arg2.
667              Currently,  the  only  permitted  value  for  this  argument  is
668              PR_SPEC_STORE_BYPASS  (otherwise  the  call fails with the error
669              ENODEV).  This setting is  a  per-thread  attribute.   The  arg3
670              argument  is  used to hand in the control value, which is one of
671              the following:
673              PR_SPEC_ENABLE
674                     The speculation feature is enabled,  mitigation  is  dis‐
675                     abled.
677              PR_SPEC_DISABLE
678                     The   speculation  feature  is  disabled,  mitigation  is
679                     enabled
681              PR_SPEC_FORCE_DISABLE
682                     Same as PR_SPEC_DISABLE but cannot be undone.   A  subse‐
683                     quent prctl(..., PR_SPEC_ENABLE) will fail with the error
684                     EPERM.
686              Any other value in arg3 will result in the call failing with the
687              error ERANGE.
689              The  arg4  and  arg5 arguments must be specified as 0; otherwise
690              the call fails with the error EINVAL.
692              The  speculation  feature  can  also  be   controlled   by   the
693              spec_store_bypass_disable  boot  parameter.   This parameter may
694              enforce a read-only policy which will result in the prctl() call
695              failing with the error ENXIO.  For further details, see the ker‐
696              nel source file Documentation/admin-guide/kernel-parameters.txt.
698       PR_SET_THP_DISABLE (since Linux 3.15)
699              Set the state of the "THP disable" flag for the calling  thread.
700              If  arg2  has  a nonzero value, the flag is set, otherwise it is
701              cleared.  Setting this flag  provides  a  method  for  disabling
702              transparent  huge  pages for jobs where the code cannot be modi‐
703              fied, and using a malloc hook with madvise(2) is not  an  option
704              (i.e., statically allocated data).  The setting of the "THP dis‐
705              able" flag is inherited by a child created via  fork(2)  and  is
706              preserved across execve(2).
708       PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)
709              Disable   all  performance  counters  attached  to  the  calling
710              process, regardless of whether the counters were created by this
711              process or another process.  Performance counters created by the
712              calling process for other processes are  unaffected.   For  more
713              information on performance counters, see the Linux kernel source
714              file tools/perf/design.txt.
716              Originally   called    PR_TASK_PERF_COUNTERS_DISABLE;    renamed
717              (retaining the same numerical value) in Linux 2.6.32.
719       PR_TASK_PERF_EVENTS_ENABLE (since Linux 2.6.31)
720              The  converse of PR_TASK_PERF_EVENTS_DISABLE; enable performance
721              counters attached to the calling process.
723              Originally called PR_TASK_PERF_COUNTERS_ENABLE; renamed in Linux
724              2.6.32.
726       PR_GET_THP_DISABLE (since Linux 3.15)
727              Return  (as the function result) the current setting of the "THP
728              disable" flag for the calling thread: either 1, if the  flag  is
729              set, or 0, if it is not.
731       PR_GET_TID_ADDRESS (since Linux 3.5)
732              Return the clear_child_tid address set by set_tid_address(2) and
733              the clone(2) CLONE_CHILD_CLEARTID flag, in the location  pointed
734              to by (int **) arg2.  This feature is available only if the ker‐
735              nel is built with the CONFIG_CHECKPOINT_RESTORE option  enabled.
736              Note  that  since the prctl() system call does not have a compat
737              implementation for the AMD64 x32 and MIPS n32 ABIs, and the ker‐
738              nel  writes  out a pointer using the kernel's pointer size, this
739              operation expects a user-space buffer of  8  (not  4)  bytes  on
740              these ABIs.
742       PR_SET_TIMERSLACK (since Linux 2.6.28)
743              Each  thread  has two associated timer slack values: a "default"
744              value, and a "current" value.  This operation sets the "current"
745              timer  slack  value for the calling thread.  arg2 is an unsigned
746              long value, then maximum "current" value is  ULONG_MAX  and  the
747              minimum  "current" value is 1.  If the nanosecond value supplied
748              in arg2 is greater than zero, then the "current" value is set to
749              this value.  If arg2 is equal to zero, the "current" timer slack
750              is reset to the thread's "default" timer slack value.
752              The "current" timer slack is used by the kernel to  group  timer
753              expirations  for  the  calling  thread  that  are  close  to one
754              another; as a consequence, timer expirations for the thread  may
755              be  up  to  the  specified  number of nanoseconds late (but will
756              never expire early).  Grouping timer expirations can help reduce
757              system power consumption by minimizing CPU wake-ups.
759              The  timer  expirations affected by timer slack are those set by
760              select(2),   pselect(2),   poll(2),   ppoll(2),   epoll_wait(2),
761              epoll_pwait(2),  clock_nanosleep(2),  nanosleep(2), and futex(2)
762              (and thus the library functions implemented via futexes, includ‐
763              ing    pthread_cond_timedwait(3),    pthread_mutex_timedlock(3),
764              pthread_rwlock_timedrdlock(3),    pthread_rwlock_timedwrlock(3),
765              and sem_timedwait(3)).
767              Timer slack is not applied to threads that are scheduled under a
768              real-time scheduling policy (see sched_setscheduler(2)).
770              When a new thread is created, the two  timer  slack  values  are
771              made  the  same  as  the "current" value of the creating thread.
772              Thereafter, a thread can adjust its "current" timer slack  value
773              via  PR_SET_TIMERSLACK.   The  "default" value can't be changed.
774              The timer slack values of init (PID 1), the ancestor of all pro‐
775              cesses,  are  50,000  nanoseconds  (50 microseconds).  The timer
776              slack value is inherited by a child created via fork(2), and  is
777              preserved across execve(2).
779              Since  Linux 4.6, the "current" timer slack value of any process
780              can be examined and  changed  via  the  file  /proc/[pid]/timer‐
781              slack_ns.  See proc(5).
783       PR_GET_TIMERSLACK (since Linux 2.6.28)
784              Return  (as the function result) the "current" timer slack value
785              of the calling thread.
787       PR_SET_TIMING (since Linux 2.6.0)
788              Set whether to use  (normal,  traditional)  statistical  process
789              timing  or  accurate  timestamp-based process timing, by passing
791              ING_TIMESTAMP  is  not  currently implemented (attempting to set
792              this mode will yield the error EINVAL).
794       PR_GET_TIMING (since Linux 2.6.0)
795              Return (as the function result) which process timing  method  is
796              currently in use.
798       PR_SET_TSC (since Linux 2.6.26, x86 only)
799              Set  the  state  of  the  flag determining whether the timestamp
800              counter can be read by the process.  Pass PR_TSC_ENABLE to  arg2
801              to  allow it to be read, or PR_TSC_SIGSEGV to generate a SIGSEGV
802              when the process tries to read the timestamp counter.
804       PR_GET_TSC (since Linux 2.6.26, x86 only)
805              Return the state of the flag determining whether  the  timestamp
806              counter can be read, in the location pointed to by (int *) arg2.
808       PR_SET_UNALIGN
809              (Only  on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15;
810              PowerPC, since Linux 2.6.18;  Alpha,  since  Linux  2.6.22;  sh,
811              since Linux 2.6.34; tile, since Linux 3.12) Set unaligned access
812              control bits to arg2.  Pass PR_UNALIGN_NOPRINT to  silently  fix
813              up  unaligned  user  accesses,  or PR_UNALIGN_SIGBUS to generate
814              SIGBUS on unaligned user access.  Alpha also supports  an  addi‐
815              tional  flag with the value of 4 and no corresponding named con‐
816              stant, which instructs kernel to not fix up  unaligned  accesses
817              (it  is analogous to providing the UAC_NOFIX flag in SSI_NVPAIRS
818              operation of the setsysinfo() system call on Tru64).
820       PR_GET_UNALIGN
821              (See PR_SET_UNALIGN for information on  versions  and  architec‐
822              tures.)   Return  unaligned access control bits, in the location
823              pointed to by (unsigned int *) arg2.


826       On   success,   PR_GET_DUMPABLE,    PR_GET_FP_MODE,    PR_GET_KEEPCAPS,
830       returns) PR_GET_SECCOMP return the nonnegative values described  above.
831       All other option values return 0 on success.  On error, -1 is returned,
832       and errno is set appropriately.


835       EACCES option is PR_SET_SECCOMP and arg2  is  SECCOMP_MODE_FILTER,  but
836              the  process  does  not have the CAP_SYS_ADMIN capability or has
837              not set  the  no_new_privs  attribute  (see  the  discussion  of
838              PR_SET_NO_NEW_PRIVS above).
840       EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is
841              not executable.
843       EBADF  option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE,  and  the  file
844              descriptor passed in arg4 is not valid.
846       EBUSY  option  is  PR_SET_MM,  arg3 is PR_SET_MM_EXE_FILE, and this the
847              second attempt to change the /proc/pid/exe symbolic link,  which
848              is prohibited.
850       EFAULT arg2 is an invalid address.
852       EFAULT option  is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, the sys‐
853              tem was built with CONFIG_SECCOMP_FILTER, and arg3 is an invalid
854              address.
856       EINVAL The value of option is not recognized.
858       EINVAL option  is  PR_MCE_KILL  or  PR_MCE_KILL_GET  or  PR_SET_MM, and
859              unused prctl() arguments were not specified as zero.
861       EINVAL arg2 is not valid value for this option.
863       EINVAL option is PR_SET_SECCOMP or PR_GET_SECCOMP, and the  kernel  was
864              not configured with CONFIG_SECCOMP.
866       EINVAL option  is  PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER, and the
867              kernel was not configured with CONFIG_SECCOMP_FILTER.
869       EINVAL option is PR_SET_MM, and one of the following is true
871              *  arg4 or arg5 is nonzero;
873              *  arg3 is greater than TASK_SIZE (the limit on the size of  the
874                 user address space for this architecture);
876              *  arg2     is     PR_SET_MM_START_CODE,     PR_SET_MM_END_CODE,
877                 PR_SET_MM_START_DATA,         PR_SET_MM_END_DATA,          or
878                 PR_SET_MM_START_STACK, and the permissions of the correspond‐
879                 ing memory area are not as required;
881              *  arg2 is PR_SET_MM_START_BRK or  PR_SET_MM_BRK,  and  arg3  is
882                 less  than  or equal to the end of the data segment or speci‐
883                 fies a value that would cause the RLIMIT_DATA resource  limit
884                 to be exceeded.
886       EINVAL option  is PR_SET_PTRACER and arg2 is not 0, PR_SET_PTRACER_ANY,
887              or the PID of an existing process.
889       EINVAL option is PR_SET_PDEATHSIG and arg2 is not a valid  signal  num‐
890              ber.
892       EINVAL option  is PR_SET_DUMPABLE and arg2 is neither SUID_DUMP_DISABLE
893              nor SUID_DUMP_USER.
895       EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.
897       EINVAL option is PR_SET_NO_NEW_PRIVS and arg2 is  not  equal  to  1  or
898              arg3, arg4, or arg5 is nonzero.
900       EINVAL option  is  PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or arg5 is
901              nonzero.
903       EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.
905       EINVAL option is PR_GET_THP_DISABLE and arg2, arg3, arg4,  or  arg5  is
906              nonzero.
908       EINVAL option is PR_CAP_AMBIENT and an unused argument (arg4, arg5, or,
909              in the case of PR_CAP_AMBIENT_CLEAR_ALL, arg3)  is  nonzero;  or
910              arg2  has  an  invalid  value;  or arg2 is PR_CAP_AMBIENT_LOWER,
911              PR_CAP_AMBIENT_RAISE, or PR_CAP_AMBIENT_IS_SET and arg3 does not
912              specify a valid capability.
915              and unused arguments to prctl() are not 0.
917       ENODEV option was PR_SET_SPECULATION_CTRL the kernel or  CPU  does  not
918              support the requested speculation misfeature.
921              and the kernel or the  CPU  does  not  support  MPX  management.
922              Check that the kernel and processor have MPX support.
924       ENXIO  option  was  PR_SET_SPECULATION_CTRL implies that the control of
925              the  selected  speculation  misfeature  is  not  possible.   See
926              PR_GET_SPECULATION_CTRL  for  the  bit fields to determine which
927              option is available.
929       EOPNOTSUPP
930              option is PR_SET_FP_MODE and arg2 has an invalid or  unsupported
931              value.
933       EPERM  option  is  PR_SET_SECUREBITS,  and the caller does not have the
934              CAP_SETPCAP capability, or tried to unset a  "locked"  flag,  or
935              tried to set a flag whose corresponding locked flag was set (see
936              capabilities(7)).
938       EPERM  option is PR_SET_SPECULATION_CTRL wherein  the  speculation  was
939              disabled  with  PR_SPEC_FORCE_DISABLE and caller tried to enable
940              it again.
942       EPERM  option     is     PR_SET_KEEPCAPS,     and     the      caller's
943              SECBIT_KEEP_CAPS_LOCKED flag is set (see capabilities(7)).
945       EPERM  option  is  PR_CAPBSET_DROP,  and  the  caller does not have the
946              CAP_SETPCAP capability.
948       EPERM  option  is  PR_SET_MM,  and  the  caller  does  not   have   the
949              CAP_SYS_RESOURCE capability.
951       EPERM  option  is  PR_CAP_AMBIENT and arg2 is PR_CAP_AMBIENT_RAISE, but
952              either the capability specified in arg3 is not  present  in  the
953              process's  permitted  and  inheritable  capability  sets, or the
954              PR_CAP_AMBIENT_LOWER securebit has been set.
956       ERANGE option  was  PR_SET_SPECULATION_CTRL   and   arg3   is   neither


960       The prctl() system call was introduced in Linux 2.1.57.


963       This  call  is  Linux-specific.   IRIX  has a prctl() system call (also
964       introduced in Linux 2.1.44 as irix_prctl  on  the  MIPS  architecture),
965       with prototype
967           ptrdiff_t prctl(int option, int arg2, int arg3);
969       and  options  to  get the maximum number of processes per user, get the
970       maximum number of processors the calling  process  can  use,  find  out
971       whether  a specified process is currently blocked, get or set the maxi‐
972       mum stack size, and so on.


975       signal(2), core(5)


978       This page is part of release 5.04 of the Linux  man-pages  project.   A
979       description  of  the project, information about reporting bugs, and the
980       latest    version    of    this    page,    can     be     found     at
981       https://www.kernel.org/doc/man-pages/.
985Linux                             2019-08-02                          PRCTL(2)